A transimpedance amplifier (TIA) is a well-known type of electronic circuit. Referring now to FIG. 1, a TIA 100 includes an operational amplifier (opamp) 105 having a gain parameter (−gm). The opamp 105 is connected in parallel to a resistor (Rf) 110. The input to the TIA 100 is a current (Δi) 115. The output of the TIA 100 is a voltage (Δvo) 120.
Referring now to FIG. 2, the opamp 105 of the TIA 100 is replaced by a current source 205 and a transistor 210 having gain −gm. The TIA 100 in FIGS. 1 and 2 is often referred to as a transconductance amplifier because it converts the input current Δi into the output voltage Δvo.
Referring now to FIG. 3, a TIA 300 converts an input voltage (Δvi) 305 into an output voltage (Δvo) 310. The TIA 300 also includes a resistor 315 that is connected to a transistor 320. The TIA 300 is typically used in applications that require relatively low bandwidth.
Referring now to FIG. 4, a TIA 400 converts an input voltage (Δvi) 405 into an output voltage (Δvo) 410. The TIA 400 includes a second opamp 415, which is connected in series to a parallel combination of a resistor (Rf) 420 and an opamp 425. The TIA 400 is typically used for applications having higher bandwidth requirements than the TIA 300.
Ordinarily, the bandwidth of the TIA is limited to a fraction of a threshold frequency fT of transistor(s) that are used in the opamp(s). In the case of a bipolar junction transistor (BJT) such as a gallium-arsenide (GaAs) transistor, the bandwidth of the TIA is approximately equal to 10%–20% of fT. For metal-oxide-semiconductor (MOS) transistor(s), the bandwidth of the TIA is typically a few percent (i.e., approximately 2%–6%) of fT.
Referring now to FIG. 5, a TIA 500 may be configured to operate differentially using two inputs of each opamp 502 and 504. One input 505 acts as a reference, in a similar manner as ground or virtual ground in a standard configuration TIA. The input voltage Δvi and the output voltage Δvo are measured as voltage differences between a reference input 505 and a second input 510. Feedback resistors 514 and 516 are connected across the inputs and the outputs of the opamp 504.
Referring now to FIG. 6, one TIA application having a relatively high bandwidth requirement is that of an optical sensor. An optical sensor circuit 600 includes the opamp 105 and the resistor 110 of the TIA 100 that are coupled with a photodiode 605. The output of the photodiode 605 is a current Iphoto 610, which acts as an input to the TIA 100.
Increasingly, applications require both high bandwidth and high gain. Examples include optical sensors, such as fiber optic receivers, and preamplifier writers for high-speed hard disk drives. Efforts to increase the gain-bandwidth product of TIAs have been made. For example, in U.S. Pat. No. 6,114,913, which are hereby incorporated by reference, a boost current is used to increase the gain-bandwidth product in the TIA. Cascading TIA stages is also used in U.S. Pat. Nos. 5,345,073 and 4,772,859, which are hereby incorporated by reference.
Other improvements to TIAs are the subject of other patents, such as U.S. Pat. Nos. 6,084,478; 6,057,738; 6,037,841; 5,646,573; 5,532,471; 5,382,920; 5,010,588; 4,914,402; 4,764,732; 4,724,315; 4,564,818; and 4,535,233, which are hereby incorporated by reference. However, improving the gain-bandwidth product of TIAs continues to be a challenge for circuit designers.